The current protection systems for electric motors are based on a set of devices developed to detect failures that may harm the electric features of motors. Among these, we can mention the most widely used systems in industrial facilities, as follows:
Overload relays: overloads are caused by blocked rotors, high frequency of maneuvers, long starts, requests above the established design limit, lack of phase and variations in tension and frequency. Such safety devices may be based on two different principles: a) bimetal overload relay, with one relay located in each feed phase of the motor, each one of them constituted of a pair of metal blades, said blades made of metals with different linear thermal dilatation coefficients, and of a firing device contained in an insulated pack with high thermal resistance; and b) electronic overload relay, which is constituted of an electronic circuit that actuates on the system due to a correlated temperature value versus resistance, whose resistance is a function of a temperature sensor coupled to the motor stator and that has a characteristic curve, wherein the electronic system converts said value output from the sensor and actuates on the system due to the temperature.
Circuit breaker: these are partly constituted of an overload relay and a coil system per phase. When an electric current determined by the coil passes, it, actuates the mechanical system to flip the contacts of the circuit breaker and cut the current flow.
Fuses: these are based on the principle that the current passing through the conductor generates heat inside, proportional to the square of its intensity, according to Joule's integral. Based on this principle, the conductor inside the fuse is dimensioned so that, under a given current level, the heat transmission effect occurs near the adiabatic system, causing the rupture of said internal conductor by melting, thus interrupting the current flow in the phase where it is installed. The main disadvantage of this system is that it does not interrupt all the phases involved in the system, but just the one where the overload occurred, and the interruption of a phase will, in fact, cause a lack of phase.
Phase-loss relays: these are used to ensure that the motor only works in the presence of electric tension in all its electric doors. Phase-loss relays are usually installed before the maneuver element, usually called contactor, and, without tension in any phase, they cut feeding to the motor.
One of the problems connected to the use of phase-loss relays is its location in relation to the motor, i.e. before the contactors. In this case, and in case of failure in any of the contactors, the phase-loss relays will continue to identify the presence of feeding tension in all phases, thus not detecting the contactor failure and therefore hampering motor protection.
Another problem concerning phase-loss relays is the fact that users frequently do not use relays that present a circuit that compares discrepancy angles between the phases to ensure the correct symmetry between the phases. As known, when there is lack of phase, a tension is induced in it. Said tension is close to the nominal tension and is proportional to the inertia of the motor and the power used by it. Therefore, such tension induced in the phase with problems may “deceive” the phase-loss relay, compromising motor operation. To overcome said inconvenience, there are relays that, by means of a complex circuitry, analyze phase discrepancy angles between the phases; however, said circuits substantially increase cost of said protection systems.
The protection systems above have, besides the features individually explained, a common disadvantage. Since all the above systems are based on the identification of a temperature increase somewhere in the motor/circuit, in certain situations, these systems are slow to respond, due to the process of heat transfer between the parts. Thus, the maximum limits of current or temperature, regarding its insulation class, may be exceeded due to the slow response from said safety devices. Particularly, there are specific applications where the measurement of temperature is fully compromised, for example in refrigeration systems. In these applications, the motor incorporated to the compression system is exposed to low temperature gases that come from the refrigeration system, which in turn make the sensors respond to the low temperature gases and not to the real motor temperature. As a consequence, the motor may be operating at high currents, but the heat is transferred to the gas before causing variations in the sensor response.
Finally, there are problems caused by the low quality of the energy supplied to the electric motor, particularly in the presence of harmonics in the feeding tension of the motor and of harmonics in the motor current. Harmonics may cause severe damage to the motor, such as overheating, pulsing torques, motor speed variation, etc. Another serious problem caused by harmonics is due to the fact that, in certain cases, one wants to correct the power factor, but, when capacitors are put in parallel with the motor, there is a considerable possibility that such capacitor enters into resonance with some harmonic component of the system, which may cause severe damage to the motor and to other electric supply loads.